The authors used a different form than the commonly used Boussinesq approximation of the axial momentum equation to model mixed convection over an inclined surface. The common Boussinesq approximation form of the governing equations was used by Chen et al.  for regular (instead of nano) fluids. The authors proceeded to present their analyses based on this uncommon form of the governing equations.
I suppose there were additional assumption(s) made to simplify the governing equations from the common Boussinesq approximation form given by Chen et al. . Please elaborate on these assumptions so that researchers not familiar with this class of problems can understand the approach readily and appreciate the analyses that followed.